You're not forced to put main character Rage in your party. While you will sometimes be forced to put a certain character in your party as part of the story development or a particular mission, you're in general free to create your own party.

The Twitter feed notes that the game's playable character cast is large, and there are some systems in place to make sure the characters who aren't in your party don't fall too far behind. We'll be getting more details on this and other areas of the battle system later.